Detection of Liver Lesions in Colorectal Cancer Patients Using 18F-FDG PET/CT Dual-Time-Point Scan Imaging

Simple Summary Fluorine-18-fluorodeoxyglucose positron emission computed tomography/computed tomography (18F-FDG PET/CT) imaging is crucial for staging, restaging, and therapeutic response assessment in colorectal cancer (CRC) patients. However, 18F-FDG is not specific for malignant lesions. Dual-time-point imaging has emerged as a promising tool to address this limitation. Our findings indicate that delayed PET/CT images significantly increased both sensitivity (87.7% vs. 100%) and specificity (94% vs. 91%) compared to standard early images. Therefore, our study demonstrated the added value of dual-time-point scans with little extra cost and radiation exposure. Abstract Objective: The aim of this study was to evaluate the diagnostic performance of dual-time-point fluorine-18-fluorodeoxyglucose positron emission computed tomography/computed tomography (18F-FDG PET/CT) compared to conventional early imaging for detecting colorectal liver metastases (CRLM) in colorectal cancer (CRC) patients. Methods: One hundred twenty-four consecutive CRC patients underwent dual-time-point imaging scans on a retrospective basis. Histopathological confirmation and/or clinical follow-up were accepted as the gold standard. Standard uptake values (SUV), signal-to-noise ratio (SNR), retention index (RI), tumor-to-normal liver ratio (TNR), and lesion sizes were measured for early and delayed PET scans. The diagnostic performance of early and delayed images was calculated on a per-patient basis and compared using McNemar’s test. Results: Among the 124 patients, 57 (46%) had CRLM, 6 (4.8%) had benign lesions, and 61 (49.2%) had no concerning lesions detected. Smaller CRLM lesions (<5 cm3) showed significantly higher uptake in the delayed scans relative to early imaging (p < 0.001). The SUV and TNR increased significantly in delayed imaging of all metastatic lesions (p < 0.001). The retention index of all CRLM was high (40.8%), especially for small lesions (54.8%). A total of 177 lesions in delayed images and 124 in standard early images were identified. In a per-patient analysis, delayed imaging had significantly higher sensitivity (100% vs. 87.7%) and specificity (91.0% vs. 94.0%) compared to early imaging (p-value = 0.04). Conclusions: The detection of liver lesions using dual-time-point PET/CT scan improves the sensitivity and specificity for the detection of colorectal liver metastasis.


Introduction
Colorectal cancer (CRC) accounts for approximately 10% of all annually diagnosed cancers and related deaths worldwide [1].Nearly 50% of CRC patients will develop colorectal liver metastases (CRLM) during their clinical course [2].The liver is the most common site of metastasis and represents an important prognostic factor for patients with CRC [3,4].The current standard of care for CRLM patients involves neoadjuvant chemotherapy and surgical resection combined with adjuvant chemotherapy when feasible, with prognosis largely dictated by the resectability of the metastatic liver disease [3,5,6].Detecting liver metastasis early has the potential to significantly improve prognosis by more accurately selecting eligible patients for curative hepatic resection and by guiding the optimal choice of treatment strategy [7,8].
Fluorine-18-fluorodeoxyglucose positron emission computed tomography/computed tomography ( 18 F-FDG PET/CT) imaging plays an important role in staging, re-staging, and therapeutic response assessment in CRC patients [9][10][11][12].However, 18 F-FDG is not specific for malignant lesions.Many benign conditions, inflammatory lesions, and infections can lead to a false-positive finding due to increased 18 F-FDG uptake [13].Due to normal intracellular glucose-6-phosphate (G6P) levels in normal tissue, glucose analogues can eventually leave normal cells with time, in contrast to malignant cells that often demonstrate abnormally low levels of G6P [14].Dual-time-point imaging has emerged as a promising tool to target these differences in physiology between benign and malignant cells, utilizing the dynamic process of 18 F-FDG uptake and clearance, as tumor cells continue to uptake 18 F-FDG in delayed imaging, while normal tissue begins clearing the tracer, improving tumor-to-background contrast [14].
Despite advancements in hardware and reconstruction algorithms in recent years, few studies have investigated the clinical value of dual-time-point 18 F-FDG PET/CT to detect CRLM.Therefore, the aim of this study was to evaluate the diagnostic performance of dual-time-point 18 F-FDG imaging compared to standard imaging for the detection of CRLM using qualitative and semi-quantitative analysis.

Patient Selection
This retrospective study included 124 consecutive patients who underwent PET/CT scans between June 2020 and January 2021.The inclusion criteria involved patients diagnosed with colorectal cancer who underwent a PET/CT scan for staging or recurrence, based on a primary assessment of the referring physician and available clinical information at the time of the imaging request.All patients had a histological diagnosis of CRC.Patients were excluded from the study if they had (1) previous history of another malignancy, (2) received any therapy in the last 30 days prior to the PET/CT study, or (3) the presence of more than seven lesions.

Reference Standard
The reference standard was based on histopathological analysis whenever available or a combination of clinical follow-up and additional cross-sectional imaging (e.g., CT, magnetic resonance imaging).All patients without a histological diagnosis were followed for up to 12 months and considered true positive if the suspicious lesions significantly regressed or increased in size during a follow-up according to the PERCIST and RECIST criteria [15].

Imaging Protocol
All examinations were performed using a Discovery IQ with 5 rings of detectors (GE Healthcare, Waukesha, WI, USA) PET/CT system with a transaxial field of view of 70 cm and pixel size of 2.73 mm with a matrix size of 256 × 256.The PET imaging system had an axial view of 26 cm per bed position with an inter-slice spacing of 3.75 mm.The patients were asked to fast for at least 6 h prior to the study.Their plasma glucose levels were checked before the injection of 18 F-FDG and ranged from 73 to 156 mg/dL.All patients received an intravenous 18 F-FDG injection of 3.7 MBq/Kg body weight (0.10 mCi/Kg) and rested in a waiting room.Initially, low-dose CT imaging was acquired using 120 Kv, 80 mA, and a slice thickness of 5 mm for attenuation correction and anatomical localization with the administration of an oral contrast agent.The standard early whole-body PET/CT was performed from skull to mid-thigh 1 h after injection (62 ± 5 min), and the scan was acquired over approximately 10 min, 2 min per bed position.The delayed emission scanning of the 1-bed position was performed 2 h after injection (122 ± 13 min) over 4 min.All patients were positioned in a supine position with elevated arms.The PET images were reconstructed using the block sequential regularized expectation maximization (BSREM) reconstruction algorithm using a penalty function of 350 (β factor) implemented by GE and known commercially as Q.Clear.

Imaging Data Analysis
The standard early and delayed PET images were evaluated independently by two nuclear medicine physicians with more than 10 years of experience each, and differences were resolved in consensus.A semi-quantitative analysis of the liver lesion uptake using SUV was performed on the GE Advantage Workstation version 4.7.For each patient, a three-dimensional volume of interest (VOI) was carefully drawn on 18 F-FDG PET/CT-fused images using a fixed threshold of 42% of the SUV max followed by a manual adjustment to exclude the uptake from adjacent tissues to the lesion [16,17].This procedure was performed for each liver lesion detected on both early and delayed scans to obtain the SUV max and SUV mean .
The normal liver tissue parameters (SUV max-liver and SUV mean-liver ) were obtained through a spherical VOI with a diameter of 3 cm (VOI ≈ 14 cm 3 ).The VOI was placed in the center of the right lobe of the liver, avoiding the major blood vessels and liver lesions, and remained the same for both early and late PET imaging.
The signal-to-noise ratio (SNR) in the liver of the standard early and delayed PET/CT scans was measured to compare the image quality of both scans.The SNR was calculated according to the methods previously reported [18,19].The SNR of the liver in the 18 F-FDG PET images was defined as the mean of the standardized uptake value (SUV) divided by the standard deviation (SD).The tumor-to-normal liver ratio (TNR) of 18 F-FDG on both the standard early and delayed scans was calculated for all liver lesions by dividing the maximum lesion SUV of the liver lesion (SUV max_lesion ) and the mean SUV of normal liver tissue (SUV mean_liver ) as shown in Equation (1).

TNR (early,delay) =
SUV max_lesion SUV mean_liver (1) The percentual SUV difference between early and delayed images of all liver lesions was determined by the retention index RI calculated as shown in Equation (2).
The semi-quantitative analysis of the CRLM lesions was performed for all lesions, lesions with volume smaller than 5 cm 3 and those larger than 5 cm 3 for early and delayed PET/CT images.

Statistical Analysis
Data were presented as a mean SD, frequency, percentage, median [IQR], and range.The Shapiro-Wilk test was used to assess the normality of the data distribution.Welch's t-test was used for normal continuous variables, or Wilcoxon signed rank sum test if normality was not met.Pearson's correlation coefficient was used to assess linear associations between continuous variables.Coefficients were interpreted using the following parameters: 0.00-0.20,very weak; 0.20-0.40,weak; 0.40-0.70,moderate; 0.70-0.90,strong; and 0.90, very strong.The sensitivity, specificity, and accuracy for early and delayed emission scanning were assessed using McNemar's test in a per-patient analysis.Statistical analyses were performed using R Statistical Software (version 4.2.3,R Core Team 2023, Vienna, Austria).

Ethics Approval
All procedures performed in the studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.The institutional ethics committee of Hospital Mae de Deus approved this study, and all patients gave written informed consent.

Patient Characteristics
The demographic and clinical characteristics of the included population are presented in Table 1.A total of 124 patients were included with a mean age of 62 ± 12 years and 53 (42.8%) females.The histological classification of the CRC patients of this study was predominantly moderately differentiated (72.6%), followed by well-differentiated (18.5%), and poorly differentiated (8.9%).The median follow-up for all patients was 12 months.

Per-Patient Performance of Early and Delayed Imaging
Among the 124 patients, 57 had CRLM, according to the reference standard.Among those, 39 patients (31.2%) had multiple liver lesions, and 24 patients (19.4%) presented only a single lesion.Delayed imaging detected all patients, while early imaging only detected 50 patients.The seven cases only detected in delayed imaging were single liver lesions, as depicted in Figure 1.All the lesions detected in early imaging were also depicted in delayed scans.The per-patient sensitivity of early and delayed PET scans was calculated as 87.7% (50/57) and 100% (57/57), respectively (Table 2).
There were four false-positive cases for metastatic liver disease in the delayed group and six false-positive cases for early imaging, with all cases attributed to a single focal liver lesion.Two false-positive cases were classified based on image follow-up as inflammatory lesions given increased uptake in early imaging with no CT correlation and resolution on follow-up.The remaining four cases were confirmed via biopsy as hepatic adenoma (n = 2), focal nodular hyperplasia (n = 1), and granulomatous disease (n = 1).The per-patient specificity of early and delayed PET was 91.0% (61/67) and 94.0% (63/67), respectively.The McNemar test revealed a significantly higher diagnostic performance of delayed images compared to standard early imaging (p = 0.04).A false-positive case of a hepatic adenoma initially interpreted as potential CRLM on both early and delayed imaging is shown in Figure 2.
hepatic adenoma initially interpreted as potential CRLM on both early and delayed imaging is shown in Figure 2.      hepatic adenoma initially interpreted as potential CRLM on both early and delayed imaging is shown in Figure 2.     of delayed imaging in a per-patient analysis, which is more relevant in the context of CRC due to the significant change in clinical staging in the presence of metastatic disease.Previous studies using a dual-time-point strategy have demonstrated an improved diagnostic performance compared to early imaging alone.The study conducted by Mao et al. showed a nearly 21% increase in the detection of small CRLM using delayed imaging [20].Dirisamer et al. also showed a 30% increase in the detection of liver metastasis using a delayed technique in a study including different primary cancers with known liver lesions [21].Moreover, a recent triple-time-point study by Yen et al. enrolling 310 patients with diverse malignancies for initial staging or recurrence demonstrated a significantly increased sensitivity and accuracy with delayed imaging [22].
The SUV mean and SUV max of normal liver parenchyma in delayed scans were significantly lower compared to early imaging in our study, improving the TNR and RI of hypermetabolic focal liver lesions.Similar results have been described in the literature [20, 23,24].Higashi et al. [25] suggested that a high retention index could predict increased HK-II expression and phosphorylation rate, conditions to accumulate 18 F-FDG in cancer cells [25].In our study, the RI of all CRLM was high, especially for small lesions.
Although dual time-point PET scan has been previously investigated by other groups, our study was the first, to our knowledge, to use the Bayesian penalized likelihood (BPL) reconstruction algorithm, which could have contributed to the improved lesion detection.BPL uses a block sequential regularized expectation maximization to improve the algorithm optimization for lesion detection, and its performance is superior to the conventional standard algorithms in small lesion detectability while improving the accuracy of the quantitative parameters [26][27][28].Parvizi et al. [29] suggested BPL may improve the diagnostic performance of 18 F-FDG PET/CT in CRLM patients.It uses a penalty function that controls noise suppression, ensuring algorithm convergence of image accuracy and improving the image quality [30].The algorithm includes point spread function (PSF) modulation that incorporates corrections related to the physical characteristics of radiation interaction in the detection crystals of the PET system.PSF reconstructed images of small lesions can be more susceptible to the Gibbs effect and consequently may overestimate its SUV values, although it improves the small-lesion detectability and spatial resolution [27,31,32].The partial volume effect (PVE) must also be considered as a factor that can affect the accuracy of the tracer concentration due to the limited spatial resolution of clinical PET systems [33,34].
The SUV values, indicative of tissue 18 F-FDG uptake, are affected by several factors: patient preparation, physiologic conditions, cross-calibration and clock synchronization between dose calibrator and PET/CT system, data accuracy (weight, height, injection time), image acquisition, and the reconstruction image algorithm.Our study carefully monitored and standardized these variables to ensure the repeatability of the SUV values.Additionally, the SNR liver measurements are influenced by low-count statistics.In our study, the administered dose (calculated per patient based on body weight) and the time from post-injection to early and delayed PET/CT scans were strictly controlled factors; therefore, its influence could be considered negligible.The SNR liver measured in our study was similar in delayed and early images; however, as an index of image quality, SNR is not considered an adequate parameter to be analyzed unattended since image quality is a combination of noise and contrast.
The delayed PET/CT image acquired 2 h after the 18 F-FDG administration was performed with a 1-bed position localized over the hepatic region to limit the radiation exposure.The incremental radiation dose is minimal using low-dose CT, while the potential benefits of improved sensitivity may have a significant impact on patient management and prognosis [35].
There are several limitations in the present study.Qualitative and semi-quantitative analysis of the 18 F-FDG PET/CT images is also affected by respiratory motion associated with physiological liver uptake.The SUV max is the most common metric used in clinical practice but is potentially more biased and very sensitive to noise since it is measured from a single voxel within the entire VOI.Although SUV max has limitations, it is more reproducible than SUV mean due to its strong dependence on VOI designing and susceptibility to PVEinduced errors [33,36].A selection bias due to the inclusion of patients highly suspected of hepatic involvement can be present in the results, and delayed imaging detected all CRLM patients; consequently, the overall sensitivity of delayed PET scans was 100%.Despite the limitations, performing an additional scan for patients suspected to have CRLM supports the potential benefit of improved diagnostic performance.

Conclusions
The detection of liver lesions in colorectal cancer patients using dual-time-point PET/CT scans improves the detection of metastatic hepatic lesions and specificity compared to the standard early PET/CT scans in patients with colorectal cancer.

Figure 1 .
Figure 1.68-year-old patient with colorectal cancer and no liver lesion detected on contrast-enhanced computed tomography.(a) Coronal MIP, axial PET, axial CT, and fused CT during the early phase did not demonstrate any hypermetabolic focal liver lesion; (b) Coronal MIP, axial PET, axial CT, and fused images during the delayed phase demonstrate a focal hypermetabolic lesion in hepatic segment 5 corresponding to a liver metastasis.
TP, true positive; TN, true negative; FP, false positive; FN, false negative.* McNemar test for the comparison of diagnostic performance of delayed vs. early imaging.

Figure 2 .
Figure 2. 61-year-old female with colorectal cancer and a single hypermetabolic liver lesion in the left lobe on both (a) delayed (SUVmax 14.3) and (b) early phases (SUVmax 13.4).Histopathological analysis confirmed the diagnosis of hepatic adenoma.

Figure 1 .
Figure 1.68-year-old patient with colorectal cancer and no liver lesion detected on contrast-enhanced computed tomography.(a) Coronal MIP, axial PET, axial CT, and fused CT during the early phase did not demonstrate any hypermetabolic focal liver lesion; (b) Coronal MIP, axial PET, axial CT, and fused images during the delayed phase demonstrate a focal hypermetabolic lesion in hepatic segment 5 corresponding to a liver metastasis.

Figure 1 .
Figure 1.68-year-old patient with colorectal cancer and no liver lesion detected on contrast-enhanced computed tomography.(a) Coronal MIP, axial PET, axial CT, and fused CT during the early phase did not demonstrate any hypermetabolic focal liver lesion; (b) Coronal MIP, axial PET, axial CT, and fused images during the delayed phase demonstrate a focal hypermetabolic lesion in hepatic segment 5 corresponding to a liver metastasis.
TP, true positive; TN, true negative; FP, false positive; FN, false negative.* McNemar test for the comparison of diagnostic performance of delayed vs. early imaging.

Figure 2 .
Figure 2. 61-year-old female with colorectal cancer and a single hypermetabolic liver lesion in the left lobe on both (a) delayed (SUVmax 14.3) and (b) early phases (SUVmax 13.4).Histopathological analysis confirmed the diagnosis of hepatic adenoma.

Figure 2 .
Figure 2. 61-year-old female with colorectal cancer and a single hypermetabolic liver lesion in the left lobe on both (a) delayed (SUV max 14.3) and (b) early phases (SUV max 13.4).Histopathological analysis confirmed the diagnosis of hepatic adenoma.

Table 2 .
Per-patient diagnostic performance of early and delayed imaging for colorectal liver metastasis.

Table 2 .
Per-patient diagnostic performance of early and delayed imaging for colorectal liver metastasis.
TP, true positive; TN, true negative; FP, false positive; FN, false negative.* McNemar test for the comparison of diagnostic performance of delayed vs. early imaging.Cancers 2023, 15, x FOR PEER REVIEW 5 of 10

Table 2 .
Per-patient diagnostic performance of early and delayed imaging for colorectal liver metastasis.